Ranges Of Animals And Plants Head North

By KEITH SCHNEIDER

Published: August 13, 1991

PELLSTON, Mich.—
THE woodland deer mouse, with its long tail, big ears and powder-white belly, once was so common in the cold forests of northern Michigan that trapping it for study was hardly more difficult than collecting moths and mosquitoes. Sometime in the last 20 to 40 years, though, the deer mouse all but vanished from the woods here, its range retreating north to Michigan's upper peninsula, 30 miles away.

By itself, the deer mouse's disappearance would not have caused much of a stir along the shores of Douglas Lake, where the University of Michigan has maintained a 10,000-acre biological field station for most of the century. But a team of Michigan researchers has documented other striking changes in the geographic distribution of a dozen other plants and animals in this region.

Ferns, fish and mammals common to the southern mixed hardwood forests of the Middle West and East are moving into northern Michigan, some of them at a pace of 10 miles annually. Meanwhile, small mammals, trees and orchid plants of the north that once were plentiful at the southern edge of their range in Michigan are rapidly slipping back into Canada, their major range.

Because the research center has been collecting data for most of this century, the scientists believe the findings reflect long-term climate change, not just recent warm years that might or might not be due to global warming. Although they concede that their work does not prove the case, their study is the first formal scientific research in the United States to determine whether documented changes in species' ranges are being caused by man-made climatic change.

"It's clear there is enough circumstantial evidence now that many scientists believe the causes of these range changes deserve an immediate and intensive investigation to determine whether it is related to global warming," said Dr. James A. Teeri, director of the Michigan Biological Station, who is coordinating the research team. "The real challenge is to separate possible global warming causes from other changes in land use due to human activities."

Measuring changes in the geographical distribution of plants and animals is often an inexact scientific business. The ranges of organisms are fluid; plants and animals aggressively take ground when conditions are sweet and withdraw in the face of fire, disease, drought and other threats. To link such changes with global warming makes the issue even more murky.

There has clearly been a warming trend in the last decade. What is at issue is the cause; it may be greenhouse gases, but scientists say it is too soon to tell. Scientists in the United States and other nations have hypothesized that the warming trend is a result of industrial and agricultural gases, chiefly carbon dioxide, methane and chlorofluorocarbons, trapping infrared energy from the earth's surface and causing the heating.

Since 1870, the average temperature of the planet has risen 1.6 degrees Fahrenheit or about 1 degree Centigrade, and if a more rapid rate of increase noted over the last two decades continues, the average temperature could climb as much as 8 degrees by the end of the 21st century, some scientists say. British and American scientists said this year that 1990 was the warmest year recorded since people began measuring the planet's surface temperature. Of the 10 warmest years recorded, all have occurred since 1973. Clues in Natural Processes

It is not yet known whether the warming trend is long term, a result of natural variability or a result of increased emissions of heat-trapping gases. Studies in the last year have sought to answer the question by looking at changes in natural processes.

Scientists have found that the snow mantle covering the Northern Hemisphere is shrinking, that the sea ice near Greenland is thinning and that the Alaskan snow melted about two weeks earlier in the 1980's than it did in the 1940's.

A study by Canadian scientists showed that the average annual temperature in a region of northwestern Ontario climbed more than 3.5 degrees Fahrenheit from the late 1960's to the mid-1980's, causing more droughts and fires and making lakes shallower and more prone to contamination. Earlier this month, scientists from the National Aeronautics and Space Administration and the United States Geological Survey said the ice cap in the Arctic Sea had shrunk 2 percent from 1978 to 1987.

But if projections of a long-term, man-made warming trend are accurate, scientists would expect to observe its results also among plants, animals and ecological systems that are affected by minute shifts in natural conditions, especially in the colder climates of the Northern Hemisphere. This is a difficult endeavor when working with living plants and animals.

Previously, ecologists have noted that the Virginia opossum is rapidly moving from its native range well into New England and northern Michigan. Armadillos, red cardinals, mockingbirds and several species of rats and mice also are moving north.

But at the same time several northern species are moving south. The least weasel, for instance, is expanding out of Nebraska and into Kansas and Missouri. 'Incredible Difficulty'